Truss rod apparatus

ABSTRACT

A truss rod apparatus, for use within the neck of a stringed musical instrument, to correct both concave and convex warping of the instrument&#39;s neck. The apparatus includes a first and second rod, the rods lying parallel to each other in a groove within the instrument&#39;s neck. The groove and truss rod apparatus are overlaid by a fingerboard. The rods are fastened together at one pair of their ends. The opposite end of the first rod is fastened to an anchor block, holding the rod axially and radially immobile. The opposite end of the second rod is threaded, engaging with internal threads of a hollow bolt. The hollow bolt itself threads with a passageway through the anchor block. Upon rotation of the bolt, the effective length of the second rod may be increased or decreased without rotating the rod, causing either a concave or convex bow in the rod pair. Where the first and second rod lengths are equal, the rods lay straight, imparting no forces on the instrument neck. The apparatus is easily replaceable without dismantling of the instrument&#39;s neck.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to stringed musical instruments and, moreparticularly, to an apparatus internal to the neck of the instrument foradjusting the curvature of the neck.

2. Description of the Prior Art

Stringed musical instruments such as banjos and guitars have been knownfor many years. In these instruments, tightly wound strings arestretched from the end of a long neck to the base of a sound box towhich the neck is fastened. Sound is produced by the vibration of thestrings when plucked or when a bow is drawn across them, the sound beingamplified by the sound box. Because the strings are stretched tightlyfrom the end of the neck to the base of the sound box, a great deal oftension is placed on the neck; the point on the sound box where the neckis attached acting as a fulcrum. Thus, in response to the stringspulling at the end of the neck, the tendency is for the neck to bowconcavely.

Particularly under humid conditions, the neck, often made of wood, willhave an even greater tendancy to bow or warp. Thus, strings which havebeen properly tensioned will gradually loosen because of the neckcurving concavely. Further, the distance between the strings and thefingerboard on the neck will increase as the bowing of the neckincreases, resulting in poor fingering of the strings. Also, because ofthe wood construction of the neck and the tension imposed upon it,breakage of the neck is a possibility.

In response to these considerations, several reinforcing and wrapcounteracting devices have been introduced. The simplest of thesedevices is in the form of a metal truss rod having threaded ends. Oneend engages an anchor, and the opposite end engages a tension nut and anassociated washer. The anchor holds the truss rod end in an axiallystationary position relative to the instrument's neck. The truss rod istensioned into a concave configuration relative to the strings and ispositioned into an accomodating groove which runs the length of theinstrument's neck. The groove includes a cylindrical bore at one end ofthe neck through which one end of the truss rod protrudes. The tensionnut washer, positioned over the end of the protruding truss rod, abuts ashoulder surrounding the bore opening and remains axially stationarythereto. The tension nut is threaded onto the protruding threaded rodend adjacent to the washer. An elongate filler strip, having the samegeneral arcuate shape of the elongate groove and bowed truss rodpositioned therein, is glued into the groove, over the truss rodenclosing the truss rod between the strip and the neck of theinstrument. Finally a fingerboard is generally glued in place over thefiller strip.

Concave bowing of the neck due to the pull of the strings is correctedby rotation of the nut about the threaded truss rod end, causing axialmovement of that rod end relative to the washer and instrument neck.Tightening of the nut decreases the effective length of the bowed trussrod between the anchor and washer, causing the rod to pull downwardly onthe ends of the neck away from the strings and to force the neckupwardly at the middle of the arcuate filler strip. The net effect ofthese forces causes the neck to bow convexly relative to the strings.The degree of this convex force is adjustable by tightening or looseningthe tension nut.

The concave bow of the neck in this simplest truss rod device generallyis limited to the degree of bow imparted to the neck during itsconstruction with the tension nut loose, coupled with the forces exertedby the tightened instrument strings in the completed instrument. This isbecause no means for increasing the effective truss rod length areprovided; the rod can only be shortened. Variations on this simple trussrod theme are exemplified by U.S. Pat. Nos. 4,074,604 to Fender (centralportion of truss rod flattened in a plane perpendicular to the bow ofthe truss rod), and 4,167,133 to Adams (rod replaced by a tensionablestrap), the disclosures of which are incorporated herein by reference.

Both concave and convex bowing of a single truss rod to correct neckwarp have been advanced in U.S. Pat. No. 3,159,072 to Burns et al, thedisclosure of which is incorporated herein by reference. In the Burnsdevice, an anchored toothed wheel-worm gear mechanism alternatelylengthens or shortens the effective length of a truss rod between thegear mechanism and a distal rod anchor, the double action providingconvex and concave bowing, respectively.

Another device is known for alternately lengthening or shorteningeffective truss rod length to achieve both concave and convex bowing bya single truss rod device lacking a worm gear mechanism. Simply, asingle rod is threaded on both ends, over each of which is threaded ananchor nut. A head is brazed or welded to one end of the rod in order toturn the rod. Thus, with the anchor nuts firmly embedded within and ateither end of the instrument's neck, the effective length of the rodbetween the anchor nuts may be increased or decreased by turning therod, by its head, thereby providing convex or concave neck bow.

While the foregoing devices ultimately achieve the result ofcounteracting convex or concave neck warp, several basic problems arisefrom their use which have not been adequately addressed. The majorproblem with the foregoing devices is their repair. Because the trussrod is compressed between the filler strip and the instrument's neck,being further overlaid by a fingerboard, dismantling of the neck isnecessary to gain access to the rod. Most frequently, the need forrepair arises due to the stripping of the threads at the truss rod end.Once the threads are stripped, the tension nut may no longer betightened. As a consequence the curvature of the neck may no longer beadjusted. Generally, once the threads on a rod are stripped, it isnecessary to replace the entire rod. However, removal of the fingerboardand filler strip in order to reach the rod is no easy task, as these aregenerally firmly glued into position on the instrument's neck. Pryingthese components loose usually results in their being damaged in someway. Further, even after the rod is exposed, it too must be pried loosefrom its groove potentially resulting in even further damage to theinstrument's neck. Such a consequence would be especially devastatingwhere the instrument being repaired is one of great value or age.

Occasionally, the rod may pull free from its anchor. Repair of thisproblem is perhaps more easily effected since the rod simply needs to bereattached to its anchor, perhaps by welding or brazing. Although thisproblem arises infrequently, it still necessitates the removal of thosecomponents which cover the anchor in order to reach it.

Another problem relating to the foregoing truss rod devices is theamount of space they take up within the instrument's neck. The neck ofthe instrument is fragile enough to begin with. Carving out grooves andspaces for rods, anchors and tension nuts removes more wood of thefragile neck than is desirable. One point of the neck especiallyvulnerable to breakage is the junction between the instrument neck andthe peghead. The peghead is a flattened structure contiguous with and atan angle to the end of the instrument's neck. Apertures in the pegheadaccomodate pegs to which the strings attach. The strings may then betensioned in order to tune the instrument simply by winding them up ontothe pegs. Due to the pulling forces exerted by the strings on thepegboard neck junction, breakage at this point is most common. Hollowingout a portion of the neck in this especially vulnerable region toaccomodate either a rod anchor or tension nut results in even greatervulnerability to breakage. Ideally there would be no hollowed outportions at this pegboard neck junction.

In truss rod devices where the rod itself must rotate in order toeffectively shorten the rod length, a problem in installation arises.Because the rod must be free to rotate within the neck, the grooveaccomodating the rod must not bind the rod and, more importantly, gluewhich attaches the fingerboard, and any filler strip to the neck mustnot adhere to the rod. While some methods for preventing the rod frombeing glued are used, such as surrounding the rod with wax paper orperiodic rotation of the rod while the glue sets, any failure of thesemethods necessitates complete dismantling of the neck in order to freeup the rod. Also, if the groove is too large for the rod, the rod willvibrate sympathetically with the string vibrations when the instrumentis being played.

Although single truss rod devices have been effective in correctingconcave warping of the neck, they have been less effective forcorrecting convex warp because of the need to impose a pretensionedconcave bow on the rod within the neck. Further, these devices aredifficult to install and repair. In order to satisfy the need for moreeffective double action truss rod devices, two addition devices havebeen proposed.

The first of these consists of a rigid sleeve along the length of theneck. Movable core elements within the sleeve cooperate with coreelements secured to the sleeve, thereby imparting deflection of thesleeve and neck. Unfortunately, such an apparatus is subject to the sameaccess for repair constraints already discussed. Further, such a deviceis necessarily complex, and consequently more difficult to manufactureand use, as well as expensive.

A second approach to the problem of effective concave and convexadjustment of neck warp involves using a truss rod apparatus having tworods, an example of which is taught by U.S. Pat. No. 3,416,399 toBaldoni, the disclosure of which is incorporated herein by reference.Here, parallel rods, threaded at each end, are tensioned into a concavebow relative to the strings. The bowed rods are held bowed by a fillerstrip within an arcuate groove along the length of the neck. Anchorssecure each rod at one end of the neck while turnbuckles, anchored atthe opposite end of the neck, engage each rod independently in order toincrease or decrease the rod's effective length. In so doing, it ispossible to counteract concave and convex bowing to the same degree aswould be achieved by having two truss rod devices of the type disclosedin the U.S. Pat. No. 3,159,072 to Burns embedded in the neck in parallelrelation. Thus, a doubled force to counteract convex and concavewarping, as well as some lateral warp correction, can be achieved byusing the Baldoni device. However, because wood grains in the instrumentneck generally run the length of the neck, lateral warp in unlikely.Further, the repair problem for the Baldoni device now is doubled as tworods having the same drawbacks as all of the previous single truss roddevices are embedded in a single neck.

Yet an additional example of known truss rod devices is a double rodassembly which may be bowed in only one direction. This device consistsof two straight parallel rods which are joined together at one end. Attheir opposite unjoined ends, the rods are attached to an anchor. Onerod is fixedly attached to the anchor while the other rod passes throughthe anchor to thread with a tension nut on the other side. By tighteningthe tension nut, the effective length of the rod with which it threadsis shortened, causing the rod pair to bow. Loosening of the tension nutwill return the rod pair to its straight configuration by returning theshortened rod to its original length. This device is installed withinthe instrument neck in the straight, untensioned condition. Thus, thenecessity of an arcuate groove is eliminated. Further, a filler strip isnow unnecessary since no structure is needed to maintain the rod in abowed condition. A fingerboard may be glued directly to the instrument'sneck in order to cover the truss rod lying within the straight neckgroove. In order to prevent glue from binding the length adjustable rod,the rods are ordinarily covered with a protective cover which allows therods to slide freely within. Further, because the rods act upon eachother in order to bow, there is need for only one anchor. By placing thedevice within the groove so that the end where the rods are joinedtogether is nearest the pegheadneck junction, very little supportivewood needs to be removed from there. This greatly increases the strengthof this region. More importantly, repair is simplified since the entiredevice within a completed neck can be easily removed from the necksimply by pulling it out from the base of the neck where an opening tothe groove is provided to accomodate the anchor. Thus, instead ofremoving the glued fingerboard and filler strip to gain access to ananchor or tension nut near the fragile pegboard neck junction, theentire apparatus can simply and quickly be removed from the sturdierbase of the instrument neck.

The major drawback of this type of apparatus is that it can correct warpin the neck by bowing in only one direction. Presumably, correction ofneck warp in an opposite direction could be achieved, but only byremoving the apparatus from the base of the neck and turning it over, sothat the bow would now be appropriate to counteract that opposite warp.Such a corrective measure is entirely inadequate for day to day use ofan instrument. The net result of using known truss rod devices is thatconcave and convex neck bow can be corrected, but significantshortcomings remain as regards installation, use, and repair.

SUMMARY OF THE INVENTION

The present invention overcomes the foregoing and other drawbacks of theprior art by providing a novel truss rod apparatus, in use positionedwithin the neck of a stringed musical instrument, for correcting bothconcave and convex warping of the instrument's neck. The apparatusoccupies little space within the neck and is easily removed or replacedwithout disassembling the neck. More importantly, both concave or convexneck warp is corrected simply and effectively by turning a single bolt.

The apparatus includes a first and second flexible metal rod, each heldparallel to each other and fastened together at one end. The free end ofthe first rod is fastened to an anchor block. The anchor block itselfdefines a passageway through which the free end of the second rod isaxially movable relative to the anchor block and first rod. Axialmovement of the second rod is accomplished by a length adjustmentmechanism whereby the length of the second rod may be alternatelyincreased or decreased relative to the first rod. Adjusting the lengthof the second rod relative to the first rod causes both of the rods tobow arcuately.

In use, the rod pair extends the length of the instrument neck, beingheld snugly within a narrow groove which also extends the length of theneck. Sleeves cover the rod pair to prevent their being bound or gluedwithin the neck. A widening of the groove, open to the outside of thebase of the neck, accomodates the anchor block. A fingerboard is affixedto the instrument neck to completely enclose the apparatus within theneck. In the preferred embodiment, the parallel rods lie in a verticalplane, perpendicular to the longitudinally planar fingerboard. Thus, anywarp of the instrument neck due to such factors as the pull of thestrings or humidity may be corrected by lengthening or shortening theadjustable rod, causing the rod pair to bow arcuately, thereby forcingthe neck to conform to the bowed shape imposed by the apparatus within.

In the preferred embodiment, the length adjusting mechanism includes ahollow bolt having internal threads and external threads. The internalthreads have a smaller pitch than the external threads. The externalthreads of the bolt engage threads within the passageway defined by theanchor block. The internal bolt threads engage threads on the end of thesecond, length adjustable rod. Thus, the effective length of the secondrod may be alternately increased or decreased, depending upon the degreeof axial bolt movement caused by the bolt's rotation within the threadedanchor block passageway.

These and other features and advantages, and a fuller understanding ofthe invention, may be had by reference to the following description andclaims, taken in conjunction with the accompanying drawings.

DESCRIPTION OF THE DRAWINGS

FIG. 1 is an exploded perspective view of a banjo, showing the truss rodapparatus according to the present invention;

FIG. 2 is a longitudinal cross-section through the neck of the banjoshowing the truss rod apparatus;

FIG. 3 is a longitudinal cross-section through the instrument neck,showing an exaggerated convex arcuate bow of the neck caused by theshortened effective length of the adjustable rod;

FIG. 4 is a longitudinal cross-section through the instrument neckshowing an exaggerated concave arcuate bow of the neck caused by theincreased effective length of the adjustable rod;

FIG. 5 is a plan view of a portion of a fingerboard which has been cutaway to show the underlying truss rod apparatus disposed within a groovein the instrument neck; and

FIG. 6 is a cross-sectional view taken along a plane indicated by line6--6 FIG. 4.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring to FIG. 1, a truss rod apparatus 50 is depicted in relation toa banjo 10, the banjo 10 being depicted only as an example of variousstringed musical instruments to which the invention may be applied.Banjo 10 includes a neck 12 attached to a circular body 14 by bolts 16.Body 14 is covered on top with a tightly stretched skin 18. Skin 18 isheld in a stretched condition by a retaining ring 20 and ring tie-downscrews. Neck 12 has a groove 24 along the length of the neck by whichthe truss rod apparatus 50 is snugly held. Fingerboard 26, withfingering frets 28, is glued in place over groove 24 and truss rodapparatus 50. Neck 12 also has peghead 30 having peg holes 32 into whichstring-tensioning pegs (not shown) are inserted.

Referring to FIGS. 2, 5, and 6, truss rod apparatus 50 is shown withingroove 24 in the instrument neck 12. Truss rod apparatus 50 includes afirst rod 52 and a second rod 54. Second rod 54 is parallel to first rod52 and joined to one end of first rod 52. An anchor block 56 is snuglyheld within a widened end of groove 24. Fixedly attached to anchor block56 is first rod 52. Anchor block 56 has a passageway 58 definedtherethrough, through which the second rod 54 is axially movable. Rods52 and 54, as well as anchor block 56, preferably are made of a metalsuch as steel.

In the preferred embodiment, axial movement of the second rod 54 isaccomplished through use of a hollow threaded bolt 60. Bolt 60 hasexternal threads 62 and internal threads 64. The threads 62 have ahigher pitch than the threads 64. The external bolt threads 62 engagethreads 66 formed by the walls of the anchor block passageway 58.Internal bolt threads 64 engage threads 68 on the end of the second rod54. Bolt 60 is preferably made of a metal softer than steel, such asbrass. Use of a softer metal helps to eliminate corrosion and bindingbetween the bolt 60 and structures with which it threads. Further, inthe event threads should strip, it is more economical to simply replacebolt 60 than either rods 52, 54 or anchor block 56. In addition to theforegoing components, the rods 52, 54 are provided with anti-frictionsleeves 70. The anti-friction sleeves 70 may be in the form ofinexpensive plastic tubing positioned on the rods 52, 54.

In operation, turning of bolt 60 causes its axial movement within theanchor block passageway 58. Because the threads 64 of bolt 60 engage thethreads 68 at the end of second rod 54, rod 54 is moved axially as bolt60 moves axially. Axial movement of rod 54 is not of the same magnitudeas that of bolt 60 due to the disparity in pitch of the threads 64 andthe threads 62. Further, rotation of bolt 60 causes it to move axiallywithin the passageway 58 and about the threads 68 at the end of rod 54.In contrast, rod 52 is moved axially while remaining radiallystationary. Sleeves 70 facilitate the movement and bowing of rods 52 and54 within neck 12.

Other methods of achieving the desired result of radial immobilityduring axial movement of rod 54 are contemplated here, for example, aball and socket arrangement. Such an arrangement would replace thethreads 68 of the end of rod 54 with a ball and the internal threads 64of bolt 60 with a socket. Thus, the ball engaged in the socket wouldprovide a means for axially moving rod 52 without its radial rotation.Various equivalent techniques will be apparent to those skilled in theart.

Referring to FIGS. 3 and 4, the action of truss rod apparatus 50 withinneck 12 of a stringed musical instrument is shown. The degree ofcurvature imparted to the neck by apparatus 50 as depicted in FIGS. 3and 4 has been exaggerated for purposes of illustration. As can be seenin FIG. 3, bolt 60 is shown extended from anchor block 56, indicatingthat the effective length of rod 54 has been shortened. As a result,rods 52 and 54 bow convexly relative to strings 34. The force of thebowed truss rods 52 and 54 is imparted to neck 12, forcing it to followsuit.

Referring to FIG. 4, bolt 60 is shown screwed into anchor block 56,indicating that the effective length of rod 54 has been increased. As aresult, rods 52 and 54 bow concavely relative to string 34. Thus, neck12 is forced to follow suit.

Thus, in order to counteract a concave warp in neck 12, an equal forcein the opposite or convex direction is imparted to the neck 12 by way ofthe truss rod apparatus 50; in the case illustrated by the FIGURES, byshortening the effective length of rod 54. Conversely, to counteract aconvex warp in neck 14, the effective length of rod 54 is increased,imparting a force in the opposite or concave direction, therebystraightening the neck.

In those cases where no forces are acting upon neck 12, or warpcounteraction is unneccesary or undesired, the simple rotation of bolt60, so as to equalize the effective length of rod 54 to the fixedeffective length of rod 52, eliminates any bowing of rods 52 and 54 andthus eliminates any forces exerted on neck 12. This is an advantage overcertain prior truss rod devices which require that the instrument beassembled with a "preloading" applied to the devices and, in turn, tothe neck. Accordingly, the present invention makes it possible to applyless stress to the neck under various conditions; longevity of theinstrument therefore is enhanced.

Should the need to repair any element of the described truss rodapparatus 50 arise, the entire apparatus 50 can be easily removed bysimply pulling it from groove 24 through the widened region of thegroove 24 which opens to the outside at the base of neck 12. Again, thisfeature is an advantage over certain prior devices which require thatvirtually the entire instrument be disassembled in order to repair orreplace the truss rod device.

Although the invention has been described in its preferred form with acertain degree of particularly, it will be understood that the presentdisclosure of the preferred embodiment has been made only by way ofexample and that various changes may be resorted to without departingfrom the true spirit and scope of the invention as hereinafter claimed.It is intended that the patent shall cover, by suitable expression inthe appended claims, whatever features of patentable novelty exist inthe invention disclosed.

What is claimed is:
 1. An apparatus for adjusting neck curvature in astringed musical instrument, comprising:(a) a first elongate memberhaving a first end and a second end; (b) a second elongate memberpositioned adjacent to and parallel to said first elongate member, thesecond elongate member having a first end and a second end, the firstend of the second elongate member being fixedly connected to the firstend of the first elongate member; (c) anchoring means to which thesecond end of the first elongate member is fixedly attached, theanchoring means being positioned at one end of the musical instrumentneck; and (d) rod length adjusting means threadedly engaged to saidanchoring means and said second end of said second elongate member, sothat upon movement of said rod length adjusting means in one directionor another, said rod length adjusting means applies a tension force or acompressive force, respectively, to said second elongate member.
 2. Theapparatus as recited in claim 1, wherein the first elongate member is aflexible rod.
 3. The apparatus as recited in claim 2, wherein the rod ismade of metal.
 4. The apparatus as recited in claim 1, wherein thesecond elongate member is a flexible rod.
 5. The apparatus as recited inclaim 4, wherein the rod is made of metal.
 6. The apparatus as recitedin claim 1, wherein the first end of the second elongate member iswelded to the first end of the first elongate member.
 7. The apparatusas recited in claim 1, wherein the anchoring means is a metal block. 8.The apparatus as recited in claim 1, further comprising anti-frictionsleeves disposed about the first and second elongate members.
 9. Anapparatus for adjusting neck curvature in a stringed musical instrument,comprising:(a) a first elongate member having a first end and a secondend; (b) a second elongate member positioned adjacent to and parallel tosaid first elongate member, the second elongate member having a firstend and a second end, the first end of the second elongate member beingfixedly connected to the first end of the first elongate member; (c)anchoring means to which the second end of the first elongate member isfixedly attached, the anchoring means being positioned at one end of themusical instrument neck; and (d) rod length adjusting means having ahollow threaded bolt having internal threads and external threads, theinternal bolt threads threadedly engageable with external threads on thesecond end of the second elongate member, the external bolt threadsthreadedly engagable with a threaded passageway through the anchoringmeans, said length adjusting means being connected to the second end ofthe second elongate member for axial movement and length adjustment ofsaid second elongate member relative to the first elongate member andthe anchoring means.
 10. The apparatus as recited in claim 9, whereinthe bolt is made of a soft metal.
 11. The apparatus as recited in claim10, wherein the soft metal is brass.
 12. The apparatus as recited inclaim 9, wherein the external bolt threads have a greater pitch than theinternal bolt threads.
 13. A truss rod apparatus, in use situated withinthe neck of a stringed musical instrument, for variably adjusting neckwarping of the instrument, comprising:(a) a first rod; (b) a second rodparallel to the first rod and joined at one end to one end of the firstrod, the free end of the second rod having threads; (c) an anchoringblock to which the first rod is joined, the block having a threadedpassageway defined therethrough, the second rod being axially movablewithin the passageway; and (d) a hollow threaded bolt, the bolt havingexternal threads and internal threads, the external threads having agreater pitch than the internal threads, the external threads threadedlyengagable with the threads in the block passageway, the internal boltthreads threadedly engagable with the threads on the free end of thesecond rod, whereby the second rod may be axially extended or retractedrelative to the block without radial movement of the rod.